There is ample seating throughout for guests, and remember they severely limit the number of people allowed in. We have been about 6 times and we RARELY are in our seats. We are snorkeling, floating, sitting in the water seats by the bar, dining, snacking, visiting with critters or birds ..... I would guess we are in our lounges about one hour the entire day. Cabana would be great if you had a group, small children that needed shade and containment, medical reasons to insure lots of shade.
Temps will be no problem. Our last trip was December and I was afraid about that because all other trips were in warm weather. I had asked once in summer and they told me it's actually better because the water where fish are is cold. In summer it feels very cold, which I agree because it would take forever for me to creep in and stay in. But cooler air makes transition easier. In December is was so much better, I walked right in and out, snorkeled forever and until my camera batteries died and would never hesitate to go the cooler times of year.
Although March in FL is not really that cool so I wouldn't worry about it.